A stepper system, used in the manufacture of IC components, requires strict exposure control to secure high resolution in forming circuit patterns. Gas discharge lasers, such as excimer lasers, are now being used as the primary illumination source; replacing the i-line mercury lamps for advanced ULSI fabrication. In such industrial applications, it is extremely important to control the energy of the laser beam pulse in order to ensure consistent processing quality is maintained for each wafer. A control parameter of particular concern is the reduction in the variance of the pulse energy for each successive pulse, and the establishment of accurate dose control.
In a typical excimer laser system used as an illumination source for a stepper, as shown in FIG. 1, the excimer laser 1 outputs an excimer laser beam L which is used by stepper 9 to perform a reduction projection exposure. An oscillator 2 of laser device 1 comprises a chamber 15, an optical resonator, and other components known to those skilled in the art. The laser chamber 15 is filled with laser gasses which are typically Kr, F.sub.2 or other known rare gas-halide combinations. Discharge voltage in the form of a pulse with a predetermined width and predetermined interval is applied across electrodes 12a and 12b to excite the gasses in the laser chamber 15 to oscillate the laser beam. The oscillated laser beam is input into the resonator and output as an effective oscillated laser beam L from a front mirror (not shown) of the resonator. Since the discharge voltage is applied as a pulse, the output laser beam L is likewise in the form of a pulse.
Part of the laser beam oscillated from oscillator 2 is sampled by a beam splitter 3 and input into an output monitor 5 through lens 4. The output monitor 5 detects the energy of laser beam L for each pulse. The pulse energy detected by output monitor 5 is supplied to the laser's output control unit 6, which generates voltage data on the basis of the pulse energy, and outputs the voltage data to a laser power source 8 so that laser power source 8 supplies the desired pulse energy to the stepper. The laser power source 8 supplies a voltage V across the electrodes in accordance with the supplied voltage data to thereby perform discharge. The voltage which causes discharge is temporarily charged to a storage unit, such as a capacitor 17, provided as part of the laser power source 8. The voltage stored in storage unit 17 is discharged by a switch such as a thyratron to initiate lasing of the gas mixture.
Output control unit 6 is connected through signal lines to a stepper control unit 6 in stepper 9 and receives a triggering signal Tr from stepper control unit 10 to cause the thyratron commute, thereby causing the laser to pulse discharge. The output control unit 6 has an internal timer which sequentially measures an interval of time between adjacent times when the output control unit 6 receives the signals Tr. A gas control unit 7 is also provided to replace part of the laser gasses consumed during laser operation to help ensure a constant laser output. Electrode temperature sensor 13 is disposed on a surface of an upper discharge electrode 12a (cathode) in laser chamber 15 to detect the surface temperature of the electrodes, and gas temperature sensor 14 is likewise disposed in chamber 15 to detect the temperature of the laser gases. Signals indicative of these respective temperatures is sent to control unit 6.
In the representative design of FIG. 1, the energy output of the laser is sampled by beam splitter 3, as previously described, and used as a basis for dose control by the laser's control unit. The energy levels measured directly from the laser via beam splitter 3 may differ from measured energy values actually received by the wafer as a result of degradation through the stepper's optics. This difference can have a significant impact, particularly as design rule features for ULSI devices reach the sub-0.4 .mu.m levels.